Specificity in the adsorption of nitrogen and water on hydroxylated and dehydroxylated silicas

Specificity in the Adsorption of Nitrogen and Water on Hydroxylated and Dehydroxylated Silicas 1 F R E D E R I C K S. BAKER 2 AND K E N N E T H S. W. SING School of Chemistry, Brunel University, Uxbridge, Middlesex, UB8 3PH, England Received Suly 16, 1975; accepted September 24, 1975 Adsorption isotherms of nitrogen (at - 196°C) and water (at 25°C) were determined on nonporous, mesoporous and microporous silicas outgassed at temperatures of 25 and 1000°C. The isotherms are analyzed by the BET, FHH, and a, methods, and the adsorption characteristics are related to the extent of surface hydroxylation and the porosity of each sample. A small change in the surface concentration of hydroxyl groups produced by exposing the nonporous silica (TK 800) to water vapor did not affect the BET-nitrogen area to any significant extent, but the uptake of water was enhanced and the low-pressure hysteresis removed. Outgassing the nonporous and mesoporous samples at 1000°C resulted in only a small change in texture. In contrast, the microporous gel underwent a drastic reduction in BET-area, although the isotherm shape and as-plot show that the micropore structure was not destroyed. Values for the fractional surface coverage, Ow, by the BET-monolayer of water are found to range between ~-~0.1 and 0.9, the lowest values being obtained with the dehydroxylated silicas (irrespective of pore size) and the highest values obtained with the partially hydroxylated, microporous gels. The FHH analysis indicates that the development of the nitrogen multilayer is not much affected by surface dehydroxylafion. INTRODUCTION

In the context of physisorption, the importance of specificity is now well established (1-3). The specific contribution for the adsorption of water vapor takes the form of field-dipole interaction or hydrogen bonding with surface hydroxyls (4-8) and its removal leads to a marked reduction in the uptake of water on a given surface, i.e., the development of hydrophobicity (9). For this reason, dehydroxylated silicas exhibit a much lower affinity for water than that shown by hydroxylated surfaces (4, 6, 9). The interpretation of water adsorption data for dehydroxylated silica is made complicated, however, by the fact that the surface can undergo slow rehydroxylation as the water vapor pressure 1 Presented at the 49th National Colloid Symposium, Potsdam, New York, June 16-18, 1975. 2 Present address: Department of Chemistry, Amherst College, Amherst, Massachusetts 01002, U.S.A.

is increased (4, 6). It is well known that the slow dissociative chemisorption gives rise to low-pressure hysteresis in the water vapor isotherm (4), but the effect of the pore structure on the water-silica interaction is still not clear. Since the nonspecific dispersion term provides a relatively small contribution to the total water-silica interaction energy, it might be anticipated that the monolayer section of the water isotherm would show little dependence on pore size, provided that the surface remains fully hydroxylated. The water adsorption data reported by Kiselev and his coworkers (4, 10) appear to confirm this idea, but the results of Mikhail and Shebl (11) and others (12, 13) indicate that certain microporous silicas have a higher affinity for water vapor than would be expected from their BET-nitrogen areas. Even with some apparently nonporous forms of hydroxylated silica,

605 Copyright ~) 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

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BAKER AND SING

the level of water adsorption has been found to vary from sample to sample (14-16) and therefore, deviates from the standard isotherm proposed by Kiselev (4). In view of this situation, it seems reasonable to suppose that further progress will depend on the availability of a range of well-defined porous and nonporous silicas. Progress has been made already in the determination of standard isotherms for nitrogen and some other adsorptives on nonporous hydroxylated silicas (17). This approach has provided a basis for the interpretation of isotherm data obtained with porous silicas (18, 19). Thus, deviations from the appropriate standard isotherm (expressed in a reduced form) have been attributed to the primary physisorption process of micropore filling or to the secondary process of capillary condensation in mesopores (3). It now seems logical to extend this method of isotherm analysis to dehydroxylated silicas, and to seek a suitable nonporous reference material for this purpose. The results of nitrogen and water vapor adsorption measurements presented in this paper were obtained with representative silicas which, in their original hydroxylated form, were studied in considerable detail in our earlier investigations (16-20). In this paper, the emphasis is placed on the changes in adsorptive properties produced by surface dehydroxylation of porous and nonporous samples. It is of particular interest to examine whether microporous silica in the dehydroxylated form retains a higher affinity for water than is shown by dehydroxylated mesoporous or nonporous silicas. MATERIALS AND METHODS The results of previous investigations (17, 21) showed that certain high-area silicas may be employed as nonporous reference adsorbents. The Degussa product T K 800 was found particularly useful and recently, this material has been selected as a standard for surface area determination (22). Electron microscopy has revealed that T K 800 (made by a plasma process) is made up of discrete spherical Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

particles with a lower degree of aggregation than is found in the normal grades of Aerosil (18). Fransil, another nonporous plasma silica, has a much lower surface area than T K 800 and has also featured in previous work (18, 21). The silica gels E and J were chosen as representative Inicroporous and mesoporous adsorbents, respectively: They are two of a series of gels specially prepared by the Unilever Chemical Development Centre, England (18). The nitrogen adsorption isotherms were determined volumetrically by means of a refined apparatus based on the design of Harris and Sing (17, 20). Springham greaseless stopcocks, pressure-sensitive transducer gauges, and a set of 12 calibrated bulbs were incorporated in the high vacuum-gas handling section of the apparatus, which was divided into two parts to allow a wide range of samples of high and low surface area to be studied. One section was designed to permit high temperature outgassing of the adsorbent (up to ll00°C). Nitrogen (99.9% purity) was slowly passed through a liquid-nitrogen cold trap before storage. Gas pressures were measured by a mercury manometer with the aid of a cathetometer (to 4-0.001 cm). The digital display of the transducer pressure provided a useful means of assessing equilibration, but the transducer gauge also could be employed for routine surface area determination (giving an accuracy in the BET area to within a few percent). The cryostat bath temperature was measured with an oxygen vapor pressure thermometer at the time of each adsorption reading, allowing an accurate value of P/po to be calculated for each point on the isotherm. The adsorption isotherms for water vapor were determined gravimetrically with a quartzspring balance (20). The tapered helical quartz spring (sensitivity of about 100 era/g; maximum load of 1 g) was surrounded by a water jacket held at 26.5 4- 0.05°C, while the silica sample bucket (weight of about 0.2 g) was maintained at 25 4- 0.01°C. To permit hightemperature outgassing (up to ll00°C), the sample section of the balance case was fabricated from transparent "Vitreosil" silica glass.

VAPOR ADSORPTION ON SILICAS Spring extension measurements were taken with a cathetometer (to 4-0.001 cm). As in previous work (20), mercury and silicone oil manometers were used for pressure measurement, but a pressure-sensitive transducer (Bell and Howell type 4-353) was also employed in the water adsorption studies described here. With the transducer gauge, water vapor pressures (to 4-6.2 X 10-3 Torr) could also be measured without risk of mercury or silicone oil vapor contamination.

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RESULTS AND DISCUSSION Representative nitrogen and water isotherms are shown in Figs. 1-7. To facilitate comparison of the isotherms, the amount adsorbed is reduced to unit surface area (/zmole m -c) on the basis of the BET-nitrogen area (i.e., SBETN in Table I, column 4). I t is recognized that for microporous adsorbents (e.g., gel E), the BET-area should be regarded as an "equivalent area" rather than a real surface area (3). Certain features of the results recorded in Figs. 1-7 and Table I are considered worthy of special note.

607

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FIG. 2. Adsorption isotherms of water vapor at 25°C on samples of TK 800-II outgassed at 25°C. Treatment of the sample of T K 800-II with water vapor (at p / p o = 0.98 for 3 days at 25°C) did not change SBETN to any significant 50

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FIO. 3. Adsorption isotherms of nitrogen at 196°C on samples of TK 800-II outgassed at 25 and 1000°C. Journal of Colloid and Interface S~ience, V o l . 55, N o . 3, J u n e 1 9 7 6

608

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FI6. 4. Adsorption isotherms of nitrogen at -- 196°C on silica gel J outgassed at 25 and 1000°C. extent, although the nitrogen isotherm (Fig. 1) has developed slight hysteresis in the capillary condensation range. The water treatment has had a much greater effect on the adsorption of water vapor (Fig. 2): The water isotherm on the original T K 800-II exhibits hysteresis over the entire range of p/Po investigated, whereas 2o

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FIG..5. Adsorption isotherms of nitrogen at -- 196°C on silica gel E ontgassed at 25 and 1000°C. Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

0

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FIG. 6. Adsorption isotherms of water vapor at 25°C on samples of TK 800-II, gel E and gel J ontgassed at 1000°C. that on the water-treated sample is reversible at P/Po< 0.6. To obtain dehydroxylated surfaces, samples of T K 800-II, gel J and gel E were each outgassed for about 2 hr at 1000°C. As expected, this treatment led to some sintering, but the decrease in SBETN was appreciable only in the case of the microporous gel E. The nitrogen isotherms on T K 800-II and gel J (in Figs. 3 and 4, respectively) show small but significant changes in shape in the monolayer range, reflecting a decrease in the adsorbentadsorbate interactions as a result of surface dehydroxylation, i.e., a reduction in the field gradient-quadrupole contribution (2). It is perhaps surprising that the nitrogen isotherm (Fig. 5) on the sample of gel E outgassed at 1000°C retains its Type I character in spite of the drastic reduction in SBETN, indicating that the micropore structure was not entirely destroyed. The water isotherms (Fig. 6) on the three silicas outgassed at 1000°C are quite different to those obtained on the original materials

VAPOR ADSORPTION:ON:SILICAS outgassed a t 25°C (Figs. 2 and 7). Each isot h e r m in Fig. 6 has pronounced hysteresis, which extends to very low P/po and the initial p a r t of the adsorption branch (at P/Po < 0.3) is characteristic of the low affinity for water shown b y d e h y d r o x y l a t e d silicas (4, 6). I t is already known t h a t slow rehydroxylation takes place when d e h y d r o x y l a t e d silicas are exposed to water vapor (4, 23) and there is little d o u b t t h a t this phenomenon is responsible for the low-pressure hysteresis associated with the water isotherms in Fig. 6 and for the removal of hysteresis after water t r e a t m e n t of T K 800-II (Fig. 2). I t m a y be noted t h a t the desorption branches of the water isotherms on gels E and J outgassed at 1000°C are similar to the corresponding p a r t of the isotherms on these gels outgassed a t 25°C. This suggests t h a t pore e m p t y i n g b y water occurs in a similar m a n n e r from the h y d r o x y l a t e d and rehydroxylated gels. Values for the fractional surface coverage 0w, b y the B E T - m o n o l a y e r of water are given in Table I, column 5. B y definition, Ow---SBETW/SBETN, where SBET w is the area ap-

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TABLE I Surface Coverage by Water Silica

Fransil TK 800-II TK 800-II TK 800-IIWc Aerosil 200 Gel J Gel 59~ Gel 8124 Gel E Gel ATM Gel SB~4 Gel 59~ Gel 81.4 TK 800-II Gel J Gel E

Porosity

Non Non Non (Non) Non Meso Meso Meso Micro Micro Micro Meso Meso Non Meso Micro

Outgassing t e m p (°C)

25,140 25 140 25 140 25 110 110 25 200 110 400 400 1000 1000 1000

SBET N ( ms g - 0

39 163 164 162 194 327 273 399 (572) (693) (396) 299 388 143 289 (150)

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a

b

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0.34 0.24 0.24 0.32 -0.41 0.50 0.49 0.74 -0.82 0.36 0.31 0.06 0.08 0.06

,~4 3.6 2.5 4.1 -3.4 6.0 6.0 1.8 3.1 11.5 3.2 3.0 ,~0 ,~0

Values of 0w calculated from BET monolayer capacity. b Values of 8w calculated from water adsorption at p/po = 0.2. c TK 800-IIW, sample of TK 800-II exposed to water vapor for 3 days at p/po = 0.98 and 25°C. Journal of Colloid and Interface Science, Vol. 55, N o . 3, J u n e 1976

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BAKER AND SING

Included in Table I are data obtained in other related investigations (12, 24) and also values of 8w (in column 6) calculated from the water uptake at p/po = 0.2. It will be noted that the latter values generally agree well with values of 0w calculated from the BET-monolayer capacities. From the results in Table I, it is evident that all the silicas studied gave a lower uptake of water at the BET-monolayer level than the amount required for close-packed monolayer coverage of the whole surface (i.e., 15.7 #mole m-2). On the basis of these results, the samples may be divided into four groups: (a) the dehydroxylated silicas (outgassed at 1000°C), which give very low values of 6w (<0.1); (b) partially hydroxylated samples of Fransil and T K 800 and gels heated at 400°C, which give low values of ew (0.2--0.4); (c) hydroxylated nonporous or mesoporous silicas, outgassed at 25 or 140°C, which give "normal" values of 8w (0.4-0.5); and (d) microporous hydroxylated gels, which give high values of 8w (> 0.7). The value 6w = 0.41 for gel J agrees reasonably well with the value 8w = 0.43 (at p/po = 0.2) calculated from the results of Kiselev (4) for water adsorption on fully hydroxylated widepore silica gel. In view of the large difference between the values of ~w for the hydroxylated and dehydroxylated silicas, it might be expected that a simple relation would exist between the surface hydroxyl group population and the monolayer uptake of water vapor. The approximate values of the hydroxyl group population NoH, shown in Table 1, column 7, have been calculated from the loss in weight given by outgassing each sample at 1000°C. The value of NoH "~ 5 corresponds approximately to one OH group to one surface Si atom and probably represents a fully hydroxylated surface of amorphous silica (25). It may be noted that the exposure of T K 800-II to water vapor increased the value of Non, but did not result in full hydroxylation: Although the initial section of the water isotherm became reversible, ew for water-treated T K 800-II is similar to the values given by the silica gels 59 and 81 Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

after partial hydroxylation by heat treatment at 400°C. The results obtained with the microporous gels are not easy to explain: There is evidently no simple relation between 8w and NoN and the high affinity for water vapor appears to be dependent on both micropore structure and surface hydroxylation. One possible explanation for the special properties of the microporous gels is that some OH groups are located within micropores into which water molecules may penetrate, but that are too narrow to accept nitrogen molecules. This would suggest that the total pore volume as assessed by water adsorption would be significantly larger than that available to nitrogen. In fact, this has not been found to be the case with gels A and E (e.g., with gel E, water adsorption at p/po = 0.95 indicates a total pore volume of 0.27 cm 3 g-1 whereas the corresponding uptake of nitrogen gives 0.30 cm 3 g-l). Infrared spectroscopic studies have shown that the hydroxyl groups in narrow pores are generally hydrogen-bonded (5, 25) and that some of these hydroxyls interact strongly with certain polar adsorbate molecules (5). The high values of 8w given by the microporous silicas suggest that hydrogen-bonded clusters of adsorbed water molecules are attached to OH groups in micropores at lower p/po than on the open surface, probably bridging the pores at favourable points. Brunauer and co-workers (16) have proposed a set of multilayer thickness isotherms (t-curves) for water vapor adsorption on various nonporous solids, including different silicas. According to Brunauer's view (26), the selection of the appropriate t-curve for pore structure analysis should be made on the basis of agreement between the corresponding surface areas S, and SBET. The results reported here do not support this approach, but instead indicate that the t-curve must be obtained on a nonporous reference surface of similar chemical structure to the porous material under investigation. To take an extreme case, it is clearly incorrect to use the t-curve for water on dehydroxylated silica in the analysis of

VAPOR ADSORPTION ON SILICAS water isotherms obtained on porous hydroxylated silicas and vice versa. The results of Zettlemoyer and co-workers (15) are of special interest. They have studied the adsorption of argon and water on samples of Cab-O-Sil and Hi-Sil before and after heat treatment. The values of the ratio SBETW/ SBETAr (i.e., equivalent to 0w) were found to lie in the range 0.23-0.40 for unheated Cab-O-Sil and in the range 1.24-1.33 for unheated Hi-Sil. Rehydration of Cab-O-Sil increased the monolayer uptake of water in a similar manner to that found with TK 800. In the light of our findings, it seems likely that the very large water uptakes per unit argonarea obtained with Hi-Sil (a precipitated grade of silica) were due to the penetration of water into very narrow pores, which were unable to accommodate argon molecules. It is of interest to note that the water isotherms on Hi-Sil appeared to be steeper in the region of low p/po (indicating higher BET c values) than those obtained in the present study. If this interpretation is correct, it of course means that Hi-Sil cannot be regarded as a truly nonporous form of silica. The suggestion by Kiselev and co-workers (10) that the initial part of the water isotherm is not sensitive to pore narrowing must be challenged. At first sight this interpretation seems consistent with the Type IV character of the water isotherm on gel E (Fig. 7) and other microporous hydroxylated silicas, which give Type I nitrogen isotherms, but the high values of 0w show that the pore size analysis is not straightforward. It seems likely that very narrow micropores are filled with water at low P/Po (micropore filling involving hydrogen bonding), whereas the wider micropores are first coated with adsorbed water and subsequently filled by capillary condensation at higher p/po. The location of the lower limit of the hysteresis loop (at p/po "~ 0.25) is probably governed by the tensile strength of the liquid adsorptive rather than by the pore size distribution (27). The fact that the reduced isotherms for water vapor on the three dehydroxylated

611

silicas (Fig. 6) follow a very similar path (adsorption curve) up to p/po "~ 0.3 shows clearly that micropore filling at low p/po does not occur with dehydroxylated silica gel E. The location of the desorption curve for this microporous gel confirms that the rehydroxylated micropores regain their high affinity for water. There seems little doubt, therefore, that specific interactions are required before micropore filling by water (at normal temperatures) can take place at low P/Po. This result is not surprizing in view of the relatively small contribution provided by the nonspecific dispersion interaction (2). We turn now to the analysis of the nitrogen isotherms on dehydroxylated silicas. The as-method, which has been described in some detail in previous papers (18-21), provides a simple means of comparing the shape of an isotherm with that of a standard obtained on a nonporous reference solid. Thus, the amount adsorbed is plotted against the reduced adsorption as, interpolated from the standard isotherm at the corresponding p/po. If the reference solid is properly chosen, deviations of the as-plot from linearity may be interpreted in terms of micropore filling or capillary condensation. To express the standard isotherm in the reduced form it has been found convenient, but not essential, to place as = 1 at P/po = 0.4. The as-method has been used already for the analysis of nitrogen isotherms on hydroxylated silicas (18) and for this purpose Fransil has been found to be a satisfactory reference solid. Thus, nitrogen as-plots for different samples of T K 800 and other grades of nonporous silica were shown to be linear over a wide range of P/po. In view of the change in isotherm character as a result of dehydroxylation, it seems logical to adopt a different standard in the application of the as-method to dehydroxylated silicas. The a~-plots in Fig. 8 are for nitrogen on gels E and J, outgassed at 1000°C. In these cases, the nitrogen isotherm on dehydroxylated T K 800-II (i.e., outgassed at 1000°C) is taken as the standard. The two ~-plots are clearly quite different in character: That for dehydroxylated gel J is characteristic of mono-

Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

612

BAKER AND SING

%

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FIG. 8. as-plots for nitrogen on gels E and J outgassed at 1000°C. layer-multilayer adsorption followed by capillary condensation in mesopores, whereas that for dehydroxylated gel E provides clear evidence for micropore filling at p/po < 0.2 and multilayer adsorption on a small external surface at p/po > 0.5. Although the BET-area is considerably decreased by outgassing gel E at 1000°C, the as analysis shows that the external area is increased slightly (from "-~1 to 13 m s g-l), while the micropore volume is decreased considerably (from about 0.30 to 0.07 cm 3 g-0. Zettlemoyer (9) has successfully exploited the Frenkel-Halsey-Hill (FHH) plot for the analysis of the multilayer region of nitrogen and water isotherms on various silicas. The F H H equation may be expressed in the form log (po/p) = k/O r,

[17

where 0 is the surface coverage and k and r are characteristic constants for a given system. It was found that nitrogen on fully hydroxylated silicas gave r = 2.75 and on dehydroxylated silicas gave lower values (e.g., r -- 2.20 and 2.48), approaching the value r = 2.12, obtained with polyethylene and other low energy surfaces. The F H H plots for the isotherms nitrogen, water, and argon on the samples of Fransil and T K 800 will be discussed in detail elsewhere, Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976

but it is appropriate here to comment on differences in the r values obtained for nitrogen adsorption on the hydroxylated and dehydroxylated silicas. The F H H plots for nitrogen gave values of r in the range 2.69-2.76 for various untreated samples of T K 800 and 2.63 for the T K 800-11 outgassed at 1000°C. The nitrogen isotherm on the water-treated T K 800-II also gave a linear F H H plot in the multilayer range, but in this case r = 2.39. At first sight, this result is puzzling and apparently in conflict with the results of Zettlemoyer. However, the most likely explanation for this low value of r is that the water treatment has produced a small degree of mesoporosity, which significantly affects the course of the nitrogen isotherm at high p/po. This appears to be confirmed by the presence of the narrow hysteresis loop in the nitrogen isotherm (Fig. 1). The F H H analysis indicates that the development of the nitrogen multilayer is not much affected by dehydroxylation of the silica surface. It therefore seems that the influence of the specific, field gradient-quadrupole interaction is mainly confined to the monolayer region of the nitrogen isotherm, unlike the situation with alpha-alumina (20). Further work on other nonporous silicas is required to confirm this interpretation.

VAPOR ADSORPTION ON SILICAS REFERENCES 1. KISELEV, A. V., Discuss. Faraday Soc. 40, 205 (1965). 2. BARRER, R. M., J. Colloid Interface Sci. 21, 415 (1966). 3. SING, K. S. W., in "Colloid Science," (D. H. Everett, Ed.), Vol. 1, p. 1. Chemical Society Specialist Periodical Report, London, 1973. 4. KISELEV, A. V., in "The Structure and Properties of Porous Materials," (D. H. Everett and F. S. Stone, Eds.), p. 195. Butterworths, London, 1958. 5. SNEER, L. R., AND WARD, J. W., J. Phys. Chem. 70, 3941 (1966). 6. YOUNG,G. J., J. Colloid Sci. 13, 67 (1958). 7. KLIER, K., SHEN, J. H., ANDZETTLEMOYER,A. C., J. Phys. Chem. 77, 1458 (1973). 8. CURTHOYS,G., DAVYDOV,V. YA., KISELEV, A. V., KISELEV, S. A., AND KUZNETSOV, B. V., Y. Colloid Interface Sci. 48, 58 (1974). 9. ZETTLE~rOYER,A. C., J. Colloid Interface Sci. 28, 343 (1968). 10. DZmGIT, O. M., KISELEV, A. V., AND MUTTIK, G. G., Kolloid Zh. 24, 15 (1962). 11. MIKrIAIL, R. SI~., AND SH~BL, F. A., Y. Colloid D,terface Sci. 34, 65 (1970). 12. SING, K. S. W., ANn MADELEY, J. D., J. Appl. Chem. 4, 365 (1954). 13. BAKER,F. S., PHILLIPS, C., AND SING, K. S. W., in "Proc. Syrup. Oxide-Electrolyte Interfaces," (R. S. Alwitt, Ed.), p. 65. The Electrochemical Society, Princeton, 1973. 14. EGOROV, M. M., ANn KISELEV, V. F., Russ. J. Phys. Chem. 40, 1069 (1966). 15. BASSETT, D. R., BOUCHER, E. A., AND ZETTLE-

16. 17.

18.

19. 20.

21.

22.

23. 24.

25. 26.

27.

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MOYER, A. C., Y. Colloid Interface Sei. 27, 649 (1968). HAGYMASSY,J., BRUNAUER,S., AND MIrdtAIL, R. Srt., J. Colloid Interface Sci. 29, 485 (1969). CARRUTHERS,J. D., CUTTING,P. A., DAY, R. E., HARRIS, M. R., MITCHELL, S. A., AND SING, K. S. W. Chem. Ind. (London) 1772 (1968). BHAMBHANI,M. R., CUTTING,P. A., SING,K. S. W., AND TURK, D. H., J. Colloid Interface Sci. 38, 109 (1972). PAYNE, D. A., SING, K. S. W., AND TURK, D. H., J. Colloid Interface Sei. 43, 287 (1973). CARRUTHERS,J. D., PAYNE,D. A., SING,K. S. W., AND STRYKER,L. J., J. Colloid Interface Sci. 36, 205 (1971). SING, K. S. W., in "Surface Area Determination," (D. H. Everett and R. H. Ottewill, Eds.), p. 25. Butterworths, London, 1970. EVERETT, D. H., PARFITT,G. D., SING, K. S. W., ANn WILSON, R., Y. Appl. Chem. Biotechnol. 24, 199 (1974). DZHIGIT, O. M., KISELEV, A. V., AND MUTTIK, G. G., Kolloid Zh. 23, 461 (1961). MIKHAIL,R. SH., NASHED,S., ANDSING, K. S. W., Proceedings of the International Symposium RILEM/IUPAC "Pore Structure and Properties of Materials," Prague, 1973. Final Report, Part II, c-157. Academia, Prague, 1974. KISELEV,A. V., Discus. Faraday Soc. 52, 14 (1971). BRUNAUER,S., in "Surface Area Determination," (D. H. Everett and R. H. Ottewill, Eds.), p. 63. Butterworths, London, 1970. EVERETT,D. H., AND HAYNES, J. M., in "Colloid Science," (D. H. Everett, Ed.), Vol. 1, p. 123. Chem. Soc. Spec. Per. Rep., London, 1973.

Journal of Colloid and Interface Science, Vol. 55, No. 3, June 1976